Portable and Reliable Surface-Enhanced Raman Scattering Silicon Chip for Signal-On Detection of Trace Trinitrotoluene Explosive in Real Systems

Vapor phase detection of explosives by surface enhanced Raman scattering under ambient conditions with metal nanogap structures

Non-invasive and passive detection of explosives in the vapor phase is advantageous for military, counter-terrorism, and homeland security applications. Detection of explosives using SERS has been an active research topic. However, the vapor pressures of most explosives are low at room temperature, and consequently, the vapor phase detection by SERS is highly challenging without intentionally heating explosive powder to increase the vapor pressure. In this work, we report the rapid and sensitive detection of 2,4,6-trinitrotoluene (TNT) and 2,4-dinitrotoluene (2,4-DNT) in the vapor phase, using a gold nanogap (AuNG) SERS substrate. The AuNG SERS substrate was fabricated with electron beam evaporation, rapid thermal annealing, and wet etching. SERS measurements were carried out with an incident power as low as 0.56 mW at 785 nm. To prevent the condensation effect, the TNT and 2,4-DNT powders inside the cuvette were taken out before inserting the nanogap substrate. Our SERS results demonstrate the feasibility of the non-invasive detection of vapor phase explosives under ambient conditions.

Rapid detection of trace nitrobenzene in water via SERS using a portable Raman spectrometer

Nitrobenzene is currently the most widely used explosive substance, and is known for its high toxicity and mutagenicity. It can cause severe environmental and water pollution, posing a risk to public health. Among various explosives analysis methods, surface-enhanced Raman spectroscopy (SERS) has the advantages of fast analysis speed, low detection cost, and easy operation, and has become one of the most promising analytical detection methods. Here, we present a portable and reliable sol-based SERS method for the detection of trace amounts of 2,4,6-trinitrotoluene (TNT) in different water bodies. The Meisenheimer complex formed by nitrobenzene and hydrazine hydrate can assemble on unmodified Au nanoparticles in a sol via Au-N bonds, enabling rapid detection of TNT in seawater, lake water, and tap water using a portable Raman spectrometer. Experimental results show that this SERS method can complete the detection within a few minutes and the detection sensitivity can reach 0.01 mg L-1, which is far lower than China's national standard of no more than 0.5 mg L-1. Furthermore, this method was also successfully applied to detect trace 2,4-dinitrotoluene (2,4-DNT) and picric acid (2,4,6-trinitrophenol) in water, demonstrating its strong applicability for on-site detection of nitrobenzene explosives.

Surface plasmonic biosensors: principles, designs and applications

Recently, surface plasmon resonance (SPR) biosensors have been widely used in environmental monitoring, food contamination detection and diagnosing medical conditions due to their superior sensitivity, label-free detection and rapid analysis speed. This paper briefly elaborates on the development history of SPR technology and introduces SPR signal sensing principles. A summary of recent applications of SPR sensors in different fields is highlighted, including their figures of merit and limitations. Finally, the personal perspectives and future development trends about sensor preparation and design are discussed in detail, which may be critical for improving the performance of SPR sensors.

High-efficiency SERS platform based on 3D porous PPDA@Au NPs as a substrate for the detection of pesticides on vegetables

Nowadays, the presence of highly toxic and persistent residues of pesticides in water and food around the world is becoming a serious problem, and so their rapid and sensitive detection is critical to human health. In this work, a 3D composite nanoparticle of porous PDA (polydopamine) microspheres and Au NPs (PPDA@Au NPs) was proposed as a SERS substrate to detect pesticides. Porous PDA as a substrate was first synthesized with F127 (Pluronic F127), dopamine hydrochloride, and 1,3,5-TMB (1,3,5-trimethylbenzene) under weakly alkaline conditions by a one-step method. Then, HAuCl4 was in situ reduced in the pores of PPDA spheres and grew sequentially for effecting the reducibility of PPDA. As a result, uniform 3D PPDA@Au NPs with "hot spots" were successfully synthesized as SERS substrates, which could effectively avoid the agglomeration of gold nanoparticles to greatly improve the sensitivity and uniformity of the SERS platform. At the same time, methyl parathion, 4-chlorophenol, and 2,4-D as representatives of pesticides were detected with the proposed PPDA@Au NP-based SERS platform, with detection limits lower than 7.26, 7.52, and 11 ng mL-1, separately. The current work presents a simple preparation method to prepare sensitive and uniform SERS platform PPDA@Au NPs, which have potential for applications in actual pesticide and drug testing.

Ultra-clean ternary Au/Ag/AgCl nanoclusters favoring cryogenic temperature-boosted broadband SERS ultrasensitive detection

Exploring multifunctional surface-enhanced Raman scattering (SERS) substrates with high sensitivity, broadband response property and reliable practicability should be required for ultrasensitive molecular detection in complex environments, which is heavily dependent on the photo-induced charge transfer (PICT) efficiency realized on the desirable nano-architectures. Herein, we introduce ultra-clean ternary Au/Ag/AgCl nanoclusters (NCs) with broadband resonance crossing the visible light to near-infrared region created by one step laser irradiation of mixed metal ion solution. Interestingly, the surface defects and interaction among these unique cluster-like ternary nanostructures would be further enhanced by thermal annealing treatment at 300°C, providing higher broadband SERS activities than the reference ternary nanoparticles under 457, 532, 633, 785, and 1064 nm wavelengths excitation. More importantly, the further promoted SERS activities of the resultant Au/Ag/AgCl NCs with achievable ∼5-fold enhancement than the initial one can be conventionally realized by simplistically declining the temperature from normal 20°C to cryogenic condition at about -196°C, due to the lower temperature-suppressed non-radiative recombination of lattice thermal phonons and photogenerated electrons. The cryogenic temperature-boosted SERS of the resultant Au/Ag/AgCl NCs enables the limit of detection (LOD) of folic acid (FA) biomolecules to be achieved as low as 10-12 M, which is obviously better than that of 10-9 M at room temperature condition. Overall, the smart Au/Ag/AgCl NCs-based broadband SERS sensor provides a new avenue for ultrasensitive biomolecular monitoring at cryogenic condition.

Surface-enhanced Raman scattering integrating with machine learning for green tea storage time identification

The rapid and accurate identification of complex samples still remains a great challenge, especially for those with similar compositions. In this work, we report an integration strategy consisting of surface-enhanced Raman scattering (SERS) and machine learning to discriminate complex and similar analytes, in this case green tea products with different storage times. Surface-functionalized Ag nanoparticles (NPs) were used as a SERS substrate to reveal the changes in the sensory components of green tea with variable storage time. Principal components analysis (PCA)-based support vector machine (SVM) classification was used to extract the key spectral features and identify green tea with different storage times. The results showed that such an integration strategy achieved high predictive accuracy on time tag discrimination for green tea. The multiclass SVM classifier successfully recognized green tea with different storage times at a prediction accuracy of 95.9%, sensitivity of 96.6%, and specificity of 98.8%. Therefore, this work illustrates that the SERS-based PCA-SVM platform might be a facile and reliable tool for the identification of complex matrices with subtle differentiations.

Effective and selective measurement of 2,4-dinitroaniline utilizing constructed fluorescent carbon dots sensor derived from vitamin B1

2,4-Dinitroaniline is a multifunctional product and has been applicated in various fields. It has absorbed much concerns due to its large consumption and occurrence in many environmental matrices. It is urgent to explore reliable and sensitive measurement technology. Present study described an outstanding fluorescence sensor constructed with carbon dots (CDs) derived from vitamin B₁. The CDs were fabricated by dealing with a facile hydrothermal process, and exhibited good water solubility, fluorescence stability and highly fluorescence quenching characteristics in presence of 2,4-dinitroaniline. The excitation and emission wavelengths of the obtained CDs were 305 and 378 nm, respectively. The sensor displayed good selectivity and sensitivity for 2,4-dinitroaniline. Good linearity can be achieved in two-stage of 0.2-3 μmol L^-1 and 3-20 μmol L^-1. The sensor earned low detection limit for 2,4-dinitroaniline down to 0.05 μmol L^-1. The sensor has been commendably employed for analysis of 2,4-dinitroaniline in real aqueous samples with spiked recoveries in the range of 95.0-99.6%. The proposed sensor provided a promising alternative for analysis of environmental pollutants or other meaningful employment.

Explosive and pollutant nitroaromatic sensing through a Cd(II) based ladder shaped 1D coordination polymer

In the existing leanings of environmental and national security issues, establishment of appropriate sensors for explosive as well as pollutant nitroaromatic compounds may be considered as one of the most prodigious job for material researchers. In the current study a new Cd(II) based 1D ladder coordination polymer (CP), [Cd(4-bpd)(3-cbn)₂]_n, has been synthesized and well characterized through single crystal X-ray diffraction analysis. Interestingly, the supramolecular assembly of this compound has efficiently identified 2,4,6-trinitrophenol through fluorescence quenching method. The Stern-Volmer coefficient (K_sv) has been calculated as 6.047 × 10³ M^-1, which can be attributed to the quenching of the emission intensity. The limit of detection (LOD) has been determined as 0.260 μM following the 3σ method along with almost 95% fluorescence intensity reduction. FESEM study revealed that the crystalline nature of the compound has been altered upon interaction with the above mentioned nitroaromatic analyte. Theoretical studies were performed to get the insight idea of fluorescence quenching mechanism which also substantiated the experimental observation. The present study can pave the way for the fabrication of future generation technology in sensor field.

Trends in Application of SERS Substrates beyond Ag and Au, and Their Role in Bioanalysis

This article compares the applications of traditional gold and silver-based SERS substrates and less conventional (Pd/Pt, Cu, Al, Si-based) SERS substrates, focusing on sensing, biosensing, and clinical analysis. In recent decades plethora of new biosensing and clinical SERS applications have fueled the search for more cost-effective, scalable, and stable substrates since traditional gold and silver-based substrates are quite expensive, prone to corrosion, contamination and non-specific binding, particularly by S-containing compounds. Following that, we briefly described our experimental experience with Si and Al-based SERS substrates and systematically analyzed the literature on SERS on substrate materials such as Pd/Pt, Cu, Al, and Si. We tabulated and discussed figures of merit such as enhancement factor (EF) and limit of detection (LOD) from analytical applications of these substrates. The results of the comparison showed that Pd/Pt substrates are not practical due to their high cost; Cu-based substrates are less stable and produce lower signal enhancement. Si and Al-based substrates showed promising results, particularly in combination with gold and silver nanostructures since they could produce comparable EFs and LODs as conventional substrates. In addition, their stability and relatively low cost make them viable alternatives for gold and silver-based substrates. Finally, this review highlighted and compared the clinical performance of non-traditional SERS substrates and traditional gold and silver SERS substrates. We discovered that if we take the average sensitivity, specificity, and accuracy of clinical SERS assays reported in the literature, those parameters, particularly accuracy (93-94%), are similar for SERS bioassays on AgNP@Al, Si-based, Au-based, and Ag-based substrates. We hope that this review will encourage research into SERS biosensing on aluminum, silicon, and some other substrates. These Al and Si based substrates may respond efficiently to the major challenges to the SERS practical application. For instance, they may be not only less expensive, e.g., Al foil, but also in some cases more selective and sometimes more reproducible, when compared to gold-only or silver-only based SERS substrates. Overall, it may result in a greater diversity of applicable SERS substrates, allowing for better optimization and selection of the SERS substrate for a specific sensing/biosensing or clinical application.

Halogen ion modified Ag NPs for ultrasensitive SERS detection of nitroaromatic explosives

Trace explosive detection has become one of the hottest topics in scientific communities because homeland security is one of the top priorities for countries all around the world. In this work, Ag NPs prepared with different reducing reagents were modified with various halogen ions for the SERS detection of nitroaromatic explosives (2,4-DNT and 2,4,6-TNT). It was proposed that halogen ions probably replaced the surface adsorbates on Ag NPs, i.e., citrate ions, and gave surface access to target analytes, which in turn enhanced the SERS signal. The LOD values for TNT and 2,4-DNT were found to be only 2 femtomoles. Given its facile and the highly sensitive process, the method that we demonstrated can serve as a promising analytical technology for the ultrasensitive SERS detection of nitroaromatic explosives.

Disordered mullite grains in a sapphire-derived fiber for high-temperature sensing

In this study, a sapphire-derived fiber (SDF)-based Fabry-Pérot interferometer (FPI) is proposed and experimentally demonstrated as a high-temperature sensor using the arc discharge crystallization process, forming a region with disordered mullite grains. This shows that the disordered mullite grains are related to the gradual temperature distribution during the arc discharge process, which results in a larger refractive index (RI) modulation of the SDF near the fusing area, forming a reflection mirror. An FPI was obtained by combining the optical fiber end facet. Considering the high-temperature resistance of the fiber, the fabricated FPI was used for high-temperature sensing. This shows that the device can operate at temperatures of up to 1200 °C with a sensitivity of 15.47 pm/°C, demonstrating that the proposed devices have potential applications in high-temperature environments.

Si@Ag@PEI substrate-based SERS sensor for rapid detection of illegally adulterated sulfur dioxide in traditional Chinese medicine

The illegal adulteration of sulfur dioxide in natural healthcare products may lead to serious health problems, which raise an urgent demand of straightforward approach for detecting sulfur dioxide. In this paper, surface-enhanced Raman scattering (SERS) sensor with sample preparation apparatus for headspace adsorption of SO₂ has been developed, which was successfully applied to detect illegal adulteration of sulfur dioxide in traditional Chinese medicine (TCM). Functional membrane substrate of Si@Ag@PEI composite was synthesized to enhance the adsorption and Raman signal of SO₂. A 3D-printed headspace extraction device was designed to adsorbed SO₂ by Si@Ag@PEI membrane after micro-extraction of TCM samples in 15 min. The content of sulfur dioxide was subsequently quantitatively measured by SERS sensor. The linear range of sensor is between 2.5 and 250 mg/kg with limit of detection of 0.25 mg/kg, which is lower than the strictest standard of Chinese Pharmacopoeia (10 mg/kg). The proposed approach was used to detect the SO₂ residue in TCMs including ginseng, Salvia miltiorrhiza, and bitter almonds. The fabricated sensor exhibited satisfied sensitivity and stability, which provide a simple approach for on-site detection of illegal adulteration of sulfur dioxide.

Recent Advances in Chemical Sensors for Soil Analysis: A Review

The continuously rising interest in chemical sensors’ applications in environmental monitoring, for soil analysis in particular, is owed to the sufficient sensitivity and selectivity of these analytical devices, their low costs, their simple measurement setups, and the possibility to perform online and in-field analyses with them. In this review the recent advances in chemical sensors for soil analysis are summarized. The working principles of chemical sensors involved in soil analysis; their benefits and drawbacks; and select applications of both the single selective sensors and multisensor systems for assessments of main plant nutrition components, pollutants, and other important soil parameters (pH, moisture content, salinity, exhaled gases, etc.) of the past two decades with a focus on the last 5 years (from 2017 to 2021) are overviewed.

Harnessing selectivity in chemical sensing via supramolecular interactions: from functionalization of nanomaterials to device applications

Chemical sensing is a strategic field of science and technology ultimately aiming at improving the quality of our lives and the sustainability of our Planet. Sensors bear a direct societal impact on well-being, which includes the quality and composition of the air we breathe, the water we drink, and the food we eat. Pristine low-dimensional materials are widely exploited as highly sensitive elements in chemical sensors, although they suffer from lack of intrinsic selectivity towards specific analytes. Here, we showcase the most recent strategies on the use of (supra)molecular interactions to harness the selectivity of suitably functionalized 0D, 1D, and 2D low-dimensional materials for chemical sensing. We discuss how the design and selection of receptors via machine learning and artificial intelligence hold a disruptive potential in chemical sensing, where selectivity is achieved by the design and high-throughput screening of large libraries of molecules exhibiting a set of affinity parameters that dictates the analyte specificity. We also discuss the importance of achieving selectivity along with other relevant characteristics in chemical sensing, such as high sensitivity, response speed, and reversibility, as milestones for true practical applications. Finally, for each distinct class of low-dimensional material, we present the most suitable functionalization strategies for their incorporation into efficient transducers for chemical sensing.

Detection of Explosives by SERS Platform Using Metal Nanogap Substrates

Detecting trace amounts of explosives to ensure personal safety is important, and this is possible by using laser-based spectroscopy techniques. We performed surface-enhanced Raman scattering (SERS) using plasmonic nanogap substrates for the solution phase detection of some nitro-based compounds, taking advantage of the hot spot at the nanogap. An excitation wavelength of 785 nm with an incident power of as low as ≈0.1 mW was used to excite the nanogap substrates. Since both RDX and PETN cannot be dissolved in water, acetone was used as a solvent. TNT was dissolved in water as well as in hexane. The main SERS peaks of TNT, RDX, and PETN were clearly observed down to the order of picomolar concentration. The variations in SERS spectra observed from different explosives can be useful in distinguishing and identifying different nitro-based compounds. This result indicates that our nanogap substrates offer an effective approach for explosives identification.

Surface-enhanced Raman scattering holography chip for rapid, sensitive and multiplexed detection of human breast cancer-associated MicroRNAs in clinical samples

MicroRNAs (miRNAs) are promising biomarkers for the early diagnosis of breast cancer. Yet, simultaneous achievement of rapid, sensitive and accurate detection of diverse miRNAs in clinical samples is still challenging due to the low abundance of miRNAs and the complex procedures of RNA extraction and separation. Herein, we develop an innovative three-dimensional (3D) surface-enhanced Raman scattering (SERS) holography sensing strategy for rapid, sensitive and multiplexed detection of human breast cancer-associated miRNAs. To establish a proof of concept, nine kinds of human breast cancer-associated miRNAs are isothermally amplified by Exonuclease (Exo) III enzyme, and the products could be spatially separated to corresponding sensing region on silicon SERS substrates. Each region has been modified with corresponding hairpin DNA probes, which are used to identify and quantify the miRNAs. Different DNA probes are labeled with different Raman reporters, which serve as "SERS tags" to incorporate spectroscopic information into computer-generated 3D SERS hologram within ~9 min. We demonstrate that 3D SERS holography chip not only achieves an ultrahigh sensitivity down to ~1 aM but also feature a high correlation with RT-qPCR in the detection of nine miRNAs in 30 clinical serum samples. This work provides a feasible tool to improve the diagnosis of breast cancer.

Toward the Limitation of Dealloying: Full Spectrum Responsive Ultralow Density Nanoporous Gold for Plasmonic Photocatalytic SERS

Plasmon-mediated chemical reaction has a great potential to create self-cleaning surface-enhanced Raman scattering (SERS) substrates. However, few works have been reported to promote this goal. Here, we report ultralow density nanoporous gold (ULDNPG) that possesses an impressive full spectrum responsive characteristic with a reflectivity lower than 5% in the waveband of 300-900 nm. ULDNPG was fabricated by a sandwich dealloying strategy from ultradilute Au-Ag solid solutions with the Au content as low as 1-5 at.%. The prepared ULDNPG presents excellent SERS properties, including high sensitivity, high uniformity, and reproducibility. The full spectrum responsive characteristic of ULDNPG leads to an obvious plasmonic photocatalytic activity. The short lifetime of the SP-excited hot carriers causes a restricted self-cleaning SERS property and a strong photothermal effect for ULDNPG structures.

Ligand-Supported Hot Electron Harvesting: Revisiting the pH-Responsive Surface-Enhanced Raman Scattering Spectrum of p-Aminothiophenol

The discussion of the surface-enhanced Raman scattering (SERS) spectra of p-aminothiophenol (PATP) and of its photocatalytic reaction product 4,4'-dimercaptoazobenzene (DMAB) is important for understanding plasmon-supported spectroscopy and catalysis. Here, SERS spectra indicate that DMAB forms also in a nonphotocatalytic reaction on silver nanoparticles. Spectra measured at low pH, in the presence of the acids HCl, H₂SO₄, HNO₃, and H₃PO₄, show that DMAB is reduced to PATP when both protons and chloride ions are present. Moreover, the successful reduction of DMAB in the presence of other, halide and nonhalide, ligands suggests a central role of these species in the reduction. As discussed, the ligands increase the efficiency of hot-electron harvesting. The pH-associated reversibility of the SERS spectrum of PATP is established as an observation of the DMAB dimer at high pH and of PATP as a product of its hot-electron reduction at low pH, in the presence of the appropriate ligand.

Ultrasensitive, Specific, and Rapid Fluorescence Turn-On Nitrite Sensor Enabled by Precisely Modulated Fluorophore Binding

The precise regulation of fluorophore binding sites in an organic probe is of great significance toward the design of fluorescent sensing materials with specific functions. In this study, a probe with specific fluorescence properties and nitrite detection ability is designed by precisely modulating benzothiazole binding sites. Only the fluorophore bond at the ortho-position of the aniline moiety can specifically recognize nitrite, which ensures that the reaction products displays a robust green emission. The unique 2-(2-amino-4-carboxyphenyl) benzothiazole (ortho-BT) shows superior nitrite detection performance, including a low detection limit (2.2 fg), rapid detection time (<5 s), and excellent specificity even in the presence of >40 types of strong redox active, colored substances, nitro compounds, and metal ions. Moreover, the probe is highly applicable for the rapid on-site and semiquantitative measurement of nitrite. The proposed probe design strategy is expected to start a new frontier for the exploration of probe design methodology.

Silver nanoribbons achieved by picosecond ablation using cylindrical focusing and SERS-based trace detection of TNT

We report the fabrication of silver nanoribbons by picosecond laser ablation of bulk silver (Ag) targets submerged in double distilled water (DDW) using a cylindrical focusing geometry. The laser ablation was performed by ∼2 picosecond laser pulses and the corresponding light sheet engendered by a cylindrical lens of focal length ∼4.5 cm. The input pulse energies employed at a wavelength ∼800 nm in the experiments were ∼1000 μJ, ∼1200 μJ, and ∼1400 μJ. In contrast to the case of ablation with spherical lenses, cylindrical lens ablation produced nanoparticles (NPs) and nanostructures (NSs) in 20% less time. The data obtained from the optical characterizations exemplify that localized surface plasmon resonance (LSPR) was observed at 406 nm, 408 nm, and 410 nm for the input energies of ∼1000 μJ, ∼1200 μJ, and ∼1400 μJ, respectively. Interestingly, it was observed that the ablation performed at an input energy of ∼1200 μJ demonstrated the fabrication of Ag nanoribbons rather than the formation of Ag NPs. Selected area electron diffraction (SAED) data of the nanoribbons recorded revealed their crystalline phase and linear morphology. Ag nanomaterials (NPs and ribbons) synthesized in these experiments were employed to detect the explosive molecules of 2,4,6-trinitrotoluene (TNT) at a concentration 25 nM using the technique of surface enhanced Raman scattering. The enhancement factor in the case of Ag nanoribbons (width of ∼20-30 nm, length of ∼0.6-2 μm), obtained using the cylindrical focussing geometry at input pulse energies of ∼1200 μJ, was estimated to be ∼10⁷ for the 1362 cm^-1 mode, corresponding to the symmetric NO₂ stretch of TNT.

Ag@WS2 quantum dots for Surface Enhanced Raman Spectroscopy: Enhanced charge transfer induced highly sensitive detection of thiram from honey and beverages

Novel SERS substrates is urgently in demand for rapid and sensitive analysis of toxic agrochemicals from food. In this work, a monodispersed tungsten disulfide quantum dots modified silver nanosphere (Ag@WS₂QD) was prepared and used as SERS substrate. Ag@WS₂QD generated uniform and stable SERS signals within 2 min, displaying great promise in "mixing and reading" detection. Compared to unmodified colloidal silver nanoparticles, 4 times higher analytical enhancement factor was found in Ag@WS₂QD. Density functional theory calculation verified the enhanced charge transfer within the coupling systems of molecule-Ag@WS₂QD. Besides, the unique surface properties are beneficial for the enrichment of specific molecule. Both the chemical extraction and enhanced charge transfer contributes to rapid and sensitive SERS detection of Ag@WS₂QD. A "mixing and reading" SERS method for thiram from honey and four kinds of juice was developed from Ag@WS₂QD, showing great promise for rapid and direct SERS detection for toxic agrochemicals and further applications.

In situ formed zinc oxide/graphitic carbon nitride nanohybrid for the electrochemical determination of 4-nitrophenol

The electrochemical determination of 4-nitrophenol using a nanohybrid consisting of glassy carbon (GC) and zinc oxide/graphitic carbon nitride (ZnO/g-CN nanosheet), is described. The ZnO/g-CN nanohybrid was in situ synthesized by chemical method and well characterized using absorption spectroscopy, X-ray diffraction, X-ray photoelectron spectroscopy and transmission electron microscopic analysis. It was observed that the nanosized ZnO particles were present inside the sheet-like g-CN nanostructure. The nanohybrid-modified electrode showed an enhanced electrocatalytic response for 4-nitrophenol reduction compared with the bare GC electrode. The assay exhibited linear ranges of 13.4-100 μM and 100-1000 μM for 4-NP determination. The limit of detection and limit of quantification were 4.0 and 13.4 μM, respectively, at the working potential of - 0.85 V. An appreciable precision was found towards the stability of the assay in the determination. It provides selectivity against inorganic and organic substances such as calcium chloride, potassium chloride, nitrobenzene, uric acid, 1-chloro,2,4-dinitrobenzene, 1-bromo,2-nitrobenzene and 1-iodo,2-nitrobenzene. The practical applicability of the assay was also checked in the analysis of real water samples and satisfactory recovery of 4-NP was found. Schematic representation of the synthesis of zinc oxide (ZnO) nanostructures incorporated graphitic carbon nitride nanosheets (g-C₃N₄ NSs) and its application in the voltammetric determination of 4-nitrophenol (4-NP) is presented. The nanohybrid assay showed selectivity among coexisting compounds and good recovery in real sample analysis.

Ultrahigh-Frequency Surface Acoustic Wave Sensors with Giant Mass-Loading Effects on Electrodes

Surface acoustic wave (SAW) devices are widely used for physical, chemical, and biological sensing applications, and their sensing mechanisms are generally based on frequency changes due to mass-loading effects at the acoustic wave propagation area between two interdigitated transducers (IDTs). In this paper, a new sensing mechanism has been proposed based on a significantly enhanced mass-loading effect generated directly on Au IDT electrodes, which enables significantly enhanced sensitivity, compared with that of conventional SAW devices. The fabricated ultrahigh-frequency SAW devices show a significant mass-loading effect on the electrodes. When the Au-electrode thickness increased from 12 to 25 nm, the Rayleigh mode resonant frequency decreased from 7.77 to 5.93 GHz, while that of the higher longitudinal leaky SAW decreased from 11.87 to 9.83 GHz. The corresponding mass sensitivity of 7309 MHz·mm²·μg^-1 (Rayleigh mode) is ∼8.9 × 10¹¹ times larger than that of a conventional quartz crystal balance (with a frequency of 5 MHz) and ∼1000 times higher than that of conventional SAW devices (with a frequency of 978 MHz). Trinitrotoluene concentration as low as 4.4 × 10^-9 M (mol·L^-1) can be detected using the fabricated SAW sensor, proving its giant mass-loading effect and ultrahigh sensitivity.

Interpol review of detection and characterization of explosives and explosives residues 2016-2019

This review paper covers the forensic-relevant literature for the analysis and detection of explosives and explosives residues from 2016-2019 as a part of the 19th Interpol International Forensic Science Managers Symposium. The review papers are also available at the Interpol website at: https://www.interpol.int/Resources/Documents#Publications.

Surface enhanced Raman scattering substrate for the detection of explosives: Construction strategy and dimensional effect

Surface-enhanced Raman spectroscopy (SERS) technology has been reported to be able to quickly and non-destructively identify target analytes. SERS substrate with high sensitivity and selectivity gave SERS technology a broad application prospect. This contribution aims to provide a detailed and systematic review of the current state of research on SERS-based explosive sensors, with particular attention to current research advances. This review mainly focuses on the strategies for improving SERS performance and the SERS substrates with different dimensions including zero-dimensional (0D) nanocolloids, one-dimensional (1D) nanowires and nanorods, two-dimensional (2D) arrays, and three-dimensional (3D) networks. The effects of elemental composition, the shape and size of metal nanoparticles, hot-spot structure and surface modification on the performance of explosive detection are also reviewed. In addition, the future development tendency and application of SERS-based explosive sensors are prospected.

Biomimetic preparation of core-shell structured surface-enhanced Raman scattering substrate with antifouling ability, good stability, and reliable quantitative capability

The fouling and stability are two most critical limiting factors for practical applications of surface-enhanced Raman scattering (SERS)-based microfluidic electrophoresis device. Herein, we present a novel biomimetic nanoengineering strategy to achieve a SERS substrate featuring antifouling ability, good stability, and reliable quantitative capability. Typically, by employing tea polyphenol as the reducing agent, the substrate made of silver core-gold shell nanostructures in situ grown on silicon wafer surface is fabricated. The core-shell nanostructures are further embedded with internal standard molecules. Remarkably, the fabricated substrate preserves distinct SERS effects, adaptable reproducibility, and reliable quantitative ability even if the substrate is incubated with 15% H₂ O₂ , 13% HNO₃ , or 10⁸  CFU/mL bacteria, or suffered from 12-day continuous vibration at 250 rpm/min in PBS buffer. As a proof-of-concept application, the DNA-functionalized substrate is capable of precise quantification of Hg²⁺ with a limit of detection down to ca. 1 pM even in sewage water.

A Review of Chinese Raman Spectroscopy Research Over the Past Twenty Years

This paper introduces the major Chinese research groups in the fields of biomedicine, food safety, environmental testing, material research, archaeological and cultural relics, gem identification, forensic science, and other research areas of Raman spectroscopy and combined methods spanning the two decades from 1997 to 2017. Briefly summarized are the research directions and contents of the major Chinese Raman spectroscopy research groups, giving researchers engaged in Raman spectroscopy research a more comprehensive understanding of the state of Chinese Raman spectroscopy research and future development trends to further develop Raman spectroscopy and its applications.

Are plasmonic optical biosensors ready for use in point-of-need applications?

Plasmonics has drawn significant attention in the area of biosensors for decades due to the unique optical properties of plasmonic resonant nanostructures. While the sensitivity and specificity of molecular detection relies significantly on the resonance conditions, significant attention has been dedicated to the design, fabrication, and optimization of plasmonic substrates. The adequate choice of materials, structures, and functionality goes hand in hand with a fundamental understanding of plasmonics to enable the development of practical biosensors that can be deployed in real life situations. Here we provide a brief review of plasmonic biosensors detailing most recent developments and applications. Besides metals, novel plasmonic materials such as graphene are highlighted. Sensors based on Surface Plasmon Resonance (SPR), Localized Surface Plasmon Resonance (LSPR), and Surface Enhanced Raman Spectroscopy (SERS) are presented and classified based on their materials and structure. In addition, most recent applications to environment monitoring, health diagnosis, and food safety are presented. Potential problems related to the implementation in such applications are discussed and an outlook is presented.

Microfluidics and Surface-Enhanced Raman Spectroscopy: A Perfect Match for New Analytical Tools

In this perspective article, we emphasize the combination of Surface-Enhanced Raman Spectroscopy (SERS) and Microfluidic devices. SERS approaches have been widely studied and used for multiple applications including trace molecules detection, in situ analysis of biological samples and monitoring or, all of them with good results, however still with limitations of the technique, for example regarding with improved precision and reproducibility. These implications can be overcome by microfluidic approaches. The resulting coupling Microfluidics - SERS (MF-SERS) has recently gained increasing attention by creating thundering opportunities for the analytical field. For this purpose, we introduce some of the strategies developed to implement SERS within microfluidic reactor along with a brief overview of the most recent MF-SERS applications for biology, health and environmental concerns. Eventually, we will discuss future research opportunities of such systems.

Mussel-inspired immobilization of silver nanoparticles toward sponge for rapid swabbing extraction and SERS detection of trace inorganic explosives

It is fairly crucial to detect inorganic explosives through a sensitive and fast method in the field of public safety, nevertheless, the high non-volatility and stability characteristics severely confine their accurate on-site detection from a real-world surface. In this work, an efficient, simple and cost effective method was developed to fabricate uniform silver nanoparticles (AgNPs) immobilized on polyurethane (PU) sponge through the in-situ reduction of polydopamine (PDA) based on mussel-inspired surface chemistry, in virtue of a large quantities catechol and amine functional groups. The formed PU@PDA@Ag sponges exhibited high SERS sensitivity, uniformity and reproducibility to 4-Aminothiophenol (4-ATP) probe molecule, and the limit of detection was calculated to be about 0.02 nmol L^-1. Moreover, these PU@PDA@Ag sponges could be served as excellent flexible SERS substrates to rapidly detect trace inorganic explosives with high collection efficiency via swabbing extraction. The detection limit for perchlorates (ClO₄^-), chlorates (ClO₃^-) and nitrates (NO₃^-) were approximately down to 0.13, 0.13 and 0.11 ng respectively. These flexible substrates not only could drastically increase the sample collection efficiency, but also enhance analytical sensitivity and reliability for inorganic explosive, and would have a great potential application in the future homeland security fields.

Developing cysteamine-modified SERS substrate for detection of acidic pigment with weak surface affinity

In this paper, we developed cysteamine-modified surface-enhanced Raman scattering (SERS) substrate for detecting detect trace amount of acidic pigment that shows weak affinity with gold nanoparticles (Au NPs). To realize sensitive and reproducible detection of pigment with weak affinity, the SERS substrate was prepared by attaching cysteamine (CA) to the Au NPs, the acidic pigment molecule could rapidly reached to the surface of Au NPs because of the formation of multi‑hydrogen-bond and electrostatic interaction between the pigment and CA molecule. The proposed method allowed us to detect five kinds of acidic pigment with a limit of 1.0 ppm, which is below the strictest safety limit. Compared with the previous methods, the advantages of the present substrate were its simple substrate preparation, high reproducibility and good universality. Furthermore, the reliable and enough accurate results had been obtained by using of the proposed substrates in the assay of trace pigment in real samples.

Dual-Amplification Strategy-Based SERS Chip for Sensitive and Reproducible Detection of DNA Methyltransferase Activity in Human Serum

Herein, we present a dual-amplification sensing strategy-based surface-enhanced Raman scattering (SERS) chip, which combines rolling circle amplification (RCA) and polyadenine (PolyA) assembly for sensitive and reproducible determination of the activity of M.SssI, a cytosine-guanine dinucleotide (CpG) methyltransferase (MTase). Typically, in the presence of M.SssI, RCA process is triggered, resulting in long, single-stranded DNA (ssDNA) fragments that are hybridized with thousands of Raman reporters of Cy3. Afterward, the resultant ssDNA fragments are conjugated to SERS-active substrates made of silver core-gold satellite nanocomposites-modified silicon wafer (Ag-Au NPs@Si), with the SERS enhancement factor of ∼5 × 10⁶. The core-satellite nanostructures are assembled relied on the strong affinity of PolyA toward gold/silver surface. Of particular significance, the developed SERS chip displays an ultrahigh sensitivity with a low limit of detection (LOD) of 2.8 × 10^-3 U/mL, which is around 2 orders of magnitude higher than most reported methods. In addition, the constructed chip features a broad detection range covering from 0.05 to 50 U/mL. Besides for the ultrahigh sensitivity and broad dynamic range, the chip also features good reproducibility (e.g., the relative standard deviation (RSD) is less than ∼12%). Taking advantages of these merits, the developed chip is feasible for accurate discrimination of M.SssI with various concentrations spiked in human serum samples with good recoveries ranging from 99.6% to 107%.

Light Concentration by Metal-Dielectric Micro-Resonators for SERS Sensing

Metal-dielectric micro/nano-composites have surface plasmon resonances in visible and near-infrared domains. Excitation of coupled metal-dielectric resonances is also important. These different resonances can allow enhancement of the electromagnetic field at a subwavelength scale. Hybrid plasmonic structures act as optical antennae by concentrating large electromagnetic energy in micro- and nano-scales. Plasmonic structures are proposed for various applications such as optical filters, investigation of quantum electrodynamics effects, solar energy concentration, magnetic recording, nanolasing, medical imaging and biodetection, surface-enhanced Raman scattering (SERS), and optical super-resolution microscopy. We present the review of recent achievements in experimental and theoretical studies of metal-dielectric micro and nano antennae that are important for fundamental and applied research. The main impact is application of metal-dielectric optical antennae for the efficient SERS sensing.

Porous Silicon Photonic Crystals Coated with Ag Nanoparticles as Efficient Substrates for Detecting Trace Explosives Using SERS

Picric acid (PA) is an organic substance widely used in industry and military, which poses a great threat to the environment and security due to its unstable, toxic, and explosive properties. Trace detection of PA is also a challenging task because of its highly acidic and anionic character. In this work, silver nanoparticles (AgNPs)-decorated porous silicon photonic crystals (PS PCs) were controllably prepared as surface-enhanced Raman scattering (SERS) substrates using the immersion plating solution. PA and Rhodamine 6G dye (R6G) were used as the analyte to explore the detection performance. As compared with single layer porous silicon, the enhancement factor of PS PCs substrates is increased to 3.58 times at the concentration of 10-6 mol/L (R6G). This additional enhancement was greatly beneficial to the trace-amount-detection of target molecules. Under the optimized assay condition, the platform shows a distinguished sensitivity with the limit of detection of PA as low as 10-8 mol/L, the linear range from 10-4 to 10-7 mol/L, and a decent reproducibility with a relative standard deviation (RSD) of ca. 8%. These results show that the AgNPs-modified PS PCs substrates could also find further applications in biomedical and environmental sensing.

Recent Developments in Spectroscopic Techniques for the Detection of Explosives

Trace detection of explosives has been an ongoing challenge for decades and has become one of several critical problems in defense science; public safety; and global counter-terrorism. As a result, there is a growing interest in employing a wide variety of approaches to detect trace explosive residues. Spectroscopy-based techniques play an irreplaceable role for the detection of energetic substances due to the advantages of rapid, automatic, and non-contact. The present work provides a comprehensive review of the advances made over the past few years in the fields of the applications of terahertz (THz) spectroscopy; laser-induced breakdown spectroscopy (LIBS), Raman spectroscopy; and ion mobility spectrometry (IMS) for trace explosives detection. Furthermore, the advantages and limitations of various spectroscopy-based detection techniques are summarized. Finally, the future development for the detection of explosives is discussed.

Ag/Au Nanoparticle-Loaded Paper-Based Versatile Surface-Enhanced Raman Spectroscopy Substrates for Multiple Explosives Detection

We present a systematic study on the fabrication, characterization of versatile, and low-cost filter paper-based surface-enhanced Raman spectroscopy (SERS) substrates loaded with salt-induced aggregated Ag/Au nanoparticles (NPs). These were demonstrated as efficient SERS substrates for the detection of multiple explosive molecules such as picric acid (5 μM), 2,4-dinitrotoluene (1 μM), and 3-nitro-1,2,4-triazol-5-one (10 μM) along with a common dye molecule (methylene blue, 5 nM). The concentrations of the dye and explosive molecules in terms of mass represent 31.98 pg, 11.45 ng, 1.82 ng, and 13.06 ng, respectively. Silver (Ag) and gold (Au) colloidal NPs were prepared by femtosecond laser (∼50 fs, 800 nm, 1 kHz) ablation of Ag/Au-target immersed in distilled water. Subsequently, the aggregated nanoparticles were achieved by mixing the pure Ag and Au NPs with different concentrations of NaCl. These aggregated NPs were characterized by UV-visible absorption and high-resolution transmission electron microscopy techniques. The SERS substrates were prepared by soaking the filter paper in aggregated NPs. The morphologies of the paper substrates were investigated using field-emission scanning electron microscopy technique. We have achieved superior enhancements with high reproducibility and sensitivity for filter paper substrates loaded with Ag/Au NPs mixed for an optimum concentration of 50 mM NaCl.

Fabrication of a self-assembled and flexible SERS nanosensor for explosive detection at parts-per-quadrillion levels from fingerprints

Apart from high sensitivity and selectivity of surface-enhanced Raman scattering (SERS)-based trace explosive detection, efficient sampling of explosive residue from real world surfaces is very important for homeland security applications. Herein, we demonstrate an entirely new SERS nanosensor fabrication approach. The SERS nanosensor was prepared by self-assembling chemically synthesized gold triangular nanoprisms (Au TNPs), which we show display strong electromagnetic field enhancements at the sharp tips and edges, onto a pressure-sensitive flexible adhesive film. Our SERS nanosensor provides excellent SERS activity (enhancement factor = ∼6.0 × 106) and limit of detection (as low as 56 parts-per-quadrillions) with high selectivity by chemometric analyses among three commonly military high explosives (TNT, RDX, and PETN). Furthermore, the SERS nanosensors present excellent reproducibility (<4.0% relative standard deviation at 1.0 μM concentration) and unprecedentedly high stability with a "shelf life" of at least 5 months. Finally, TNT and PETN were analyzed and quantified by transferring solid explosive residues from fingerprints left on solid surfaces to the SERS nanosensor. Taken together, the demonstrated sensitivity, selectivity, and reliability of the measurements as well as with the excellent shelf life of our SERS nanosensors obviate the need for complicated sample processing steps required for other analytical techniques, and thus these nanosensors have tremendous potential not only in the field of measurement science but also for homeland security applications to combat acts of terror and military threats.